1. Field of the Invention
The invention is related to field of semiconductor wafer fabrication systems and more specifically, to an apparatus and method for reducing particulate contaminants from adhering to or diffusing into a semiconductor substrate being processed on the apparatus.
2. Description of the Background Art
In the field of integrated circuit fabrication and manufacturing, chemical vapor deposition (CVD) is a well established technique for depositing thin films on semiconductor substrates (i.e. a silicon wafer). Typically, the wafer is introduced to a process chamber and the wafer is heated to a desired temperature to initiate the deposition process. Specifically, the wafer is placed upon a pedestal heater. The pedestal heater contains one or more electrodes connected to a power source. When the power source is activated, a current is passed through the electrodes thereby heating the pedestal and subsequently the wafer.
A combination of inert carrier and reactant gases are introduced to the chamber. The elevated wafer temperature causes the reactive gases to break down on the wafer surface thereby depositing the desired film on the wafer surface. For example, the chemical vapor deposition of copper is achieved by using a precursor (reactant) known as CUPRASELECT.RTM. upraselect, which has the formula Cu(hfac)L. The L represents a liquid base compound containing trimethylvinylsilane (TMVS). The (hfac) represents hexafluoroacetylacetonato, and Cu represents copper. During the CVD of copper, the precursor is vaporized and flowed with a carrier gas such as Argon into a deposition chamber containing a wafer. In the chamber, the precursor is infused with thermal energy at the wafer's surface, and the following reaction results: EQU 2 Cu(hfac)L.fwdarw.Cu+Cu(hfac).sub.2 +2L (Eqn. 1)
The resulting copper (Cu) deposits on the upper surface of the wafer, along with the Cu(hfac).sub.2 byproduct. The gaseous Lewis base byproduct (2L) is purged from the chamber. To maintain the desired chemical reaction, the desired temperature in the chamber and at the wafer surface must be maintained. Accordingly, the wafer is usually in direct contact at all time with the pedestal heater.
With the wafer in contact with the pedestal heater at all times, wafer processing can be negatively affected. For example at high temperature, the pedestal heater, usually aluminum, has a high coefficient of friction relative to the backside of the silicon wafer. Should the wafer shift on the pedestal heater, for example during transfer into and out of the chamber, the backside of the wafer would be scratched. At the elevated temperatures under which chemical vapor deposition occurs, scratching releases aluminum from the pedestal heater surface which then diffuses into the silicon of the wafer. Accordingly, unexpected contaminant particles are introduced into the silicon wafer. For example, conductive particles can short the semiconductor devices, i.e. gate structures that are created on the wafer surface. Likewise, non-conducting particles can increase the resistivity of conductive layers thereby degrading performance of the device.
Other conditions can also affect wafer processing. The inert and reactive gases can also leak into the lower regions of a process chamber during the fabrication process. If this condition occurs, deposition particles may undesirably form on the pedestal heater or on the backside of the wafer. As such, the wafer is further contaminated as well as the chamber components. Once contaminated, the wafer can transfer the contaminant particles to other chambers and/or a clean wafer entering the CVD chamber can become contaminated by the improperly coated chamber components. Additionally, if a wafer is not centered on the pedestal heater properly, the edge exclusion zone about the periphery of the wafer will vary. The edge exclusion zone is defined as the edge of the wafer which is not subjected to the semiconductor wafer fabrication process. The variations in the edge exclusion zone will ultimately lead to lower yield of the wafer.
Therefore, there is a need in the art for an apparatus and method of thin film deposition via CVD that can repeatably center (axially align) wafers on the pedestal heater prior to the deposition process as well as prevent backside scratching of the wafer and resultant diffusion of contaminant particles.